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  AQS之CyclicBarrier&amp;ReentrantReadWriteLock
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<h2 id="CyclicBarrier使用及源码分析"><a href="#CyclicBarrier使用及源码分析" class="headerlink" title="CyclicBarrier使用及源码分析"></a>CyclicBarrier使用及源码分析</h2><h3 id="1、CyclicBarrier的使用"><a href="#1、CyclicBarrier的使用" class="headerlink" title="1、CyclicBarrier的使用"></a>1、CyclicBarrier的使用</h3><p>1)先看一个例子</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">CyclicBarrierTest</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">        CyclicBarrier cyclicBarrier = <span class="keyword">new</span> CyclicBarrier(<span class="number">3</span>);</span><br><span class="line"></span><br><span class="line">        <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; <span class="number">5</span>; i++) &#123;</span><br><span class="line">            <span class="keyword">new</span> Thread(<span class="keyword">new</span> Runnable() &#123;</span><br><span class="line">                <span class="meta">@Override</span></span><br><span class="line">                <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>&#123;</span><br><span class="line">                    <span class="keyword">try</span> &#123;</span><br><span class="line">                        System.out.println(Thread.currentThread().getName()</span><br><span class="line">                                + <span class="string">&quot;开始等待其他线程&quot;</span>);</span><br><span class="line">                        cyclicBarrier.await();</span><br><span class="line">                        System.out.println(Thread.currentThread().getName() + <span class="string">&quot;开始执行&quot;</span>);</span><br><span class="line">                        <span class="comment">//TODO 模拟业务处理</span></span><br><span class="line">                        Thread.sleep(<span class="number">5000</span>);</span><br><span class="line">                        System.out.println(Thread.currentThread().getName() + <span class="string">&quot;执行完毕&quot;</span>);</span><br><span class="line"></span><br><span class="line">                    &#125; <span class="keyword">catch</span> (Exception e) &#123;</span><br><span class="line">                        e.printStackTrace();</span><br><span class="line">                    &#125;</span><br><span class="line">                &#125;</span><br><span class="line">            &#125;).start();</span><br><span class="line"></span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line">执行结果：</span><br><span class="line">Thread-<span class="number">0</span>开始等待其他线程</span><br><span class="line">Thread-<span class="number">4</span>开始等待其他线程</span><br><span class="line">Thread-<span class="number">3</span>开始等待其他线程</span><br><span class="line">Thread-<span class="number">1</span>开始等待其他线程</span><br><span class="line">Thread-<span class="number">2</span>开始等待其他线程</span><br><span class="line">Thread-<span class="number">4</span>开始执行</span><br><span class="line">Thread-<span class="number">3</span>开始执行</span><br><span class="line">Thread-<span class="number">0</span>开始执行</span><br><span class="line">Thread-<span class="number">0</span>执行完毕</span><br><span class="line">Thread-<span class="number">4</span>执行完毕</span><br><span class="line">Thread-<span class="number">3</span>执行完毕</span><br></pre></td></tr></table></figure>

<p>由例子可见 CyclicBarrier 可以让一组线程全部同时执行</p>
<blockquote>
<p>字面意思回环栅栏（循环屏障），通过它可以实现让一组线程等待至某个状态（屏障点）之后再全部同时执行。叫做回环是因为当所有等待线程都被释放以后，CyclicBarrier可以被重用。</p>
</blockquote>
<p>2)在看一个例子</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//保存每个学生的平均成绩</span></span><br><span class="line">    <span class="keyword">private</span> ConcurrentHashMap&lt;String, Integer&gt; map=<span class="keyword">new</span> ConcurrentHashMap&lt;String,Integer&gt;();</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> ExecutorService threadPool= Executors.newFixedThreadPool(<span class="number">3</span>);</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> CyclicBarrier cb=<span class="keyword">new</span> CyclicBarrier(<span class="number">3</span>,()-&gt;&#123;</span><br><span class="line">        <span class="keyword">int</span> result=<span class="number">0</span>;</span><br><span class="line">        Set&lt;String&gt; set = map.keySet();</span><br><span class="line">        <span class="keyword">for</span>(String s:set)&#123;</span><br><span class="line">            result+=map.get(s);</span><br><span class="line">        &#125;</span><br><span class="line">        System.out.println(<span class="string">&quot;三人平均成绩为:&quot;</span>+(result/<span class="number">3</span>)+<span class="string">&quot;分&quot;</span>);</span><br><span class="line">    &#125;);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">count</span><span class="params">()</span></span>&#123;</span><br><span class="line">        <span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">0</span>;i&lt;<span class="number">3</span>;i++)&#123;</span><br><span class="line">            threadPool.execute(<span class="keyword">new</span> Runnable()&#123;</span><br><span class="line"></span><br><span class="line">                <span class="meta">@Override</span></span><br><span class="line">                <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>&#123;</span><br><span class="line">                    <span class="comment">//获取学生平均成绩</span></span><br><span class="line">                    <span class="keyword">int</span> score=(<span class="keyword">int</span>)(Math.random()*<span class="number">40</span>+<span class="number">60</span>);</span><br><span class="line">                    map.put(Thread.currentThread().getName(), score);</span><br><span class="line">                    System.out.println(Thread.currentThread().getName()</span><br><span class="line">                            +<span class="string">&quot;同学的平均成绩为：&quot;</span>+score);</span><br><span class="line">                    <span class="keyword">try</span> &#123;</span><br><span class="line">                        <span class="comment">//执行完运行await(),等待所有学生平均成绩都计算完毕</span></span><br><span class="line">                        cb.await();</span><br><span class="line">                    &#125; <span class="keyword">catch</span> (InterruptedException | BrokenBarrierException e) &#123;</span><br><span class="line">                        e.printStackTrace();</span><br><span class="line">                    &#125;</span><br><span class="line">                &#125;</span><br><span class="line"></span><br><span class="line">            &#125;);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        CyclicBarrierTest2 cb=<span class="keyword">new</span> CyclicBarrierTest2();</span><br><span class="line">        cb.count();</span><br><span class="line">    &#125;</span><br><span class="line">执行结果：</span><br><span class="line">pool-<span class="number">1</span>-thread-<span class="number">1</span>同学的平均成绩为：<span class="number">93</span></span><br><span class="line">pool-<span class="number">1</span>-thread-<span class="number">2</span>同学的平均成绩为：<span class="number">68</span></span><br><span class="line">pool-<span class="number">1</span>-thread-<span class="number">3</span>同学的平均成绩为：<span class="number">95</span></span><br><span class="line">三人平均成绩为:<span class="number">85</span>分</span><br></pre></td></tr></table></figure>

<p>从结果可以看到，CyclicBarrier可以实现和countDownLatch一样的功能，即等待一组线程执行完之后在执行某个汇总任务。</p>
<p>3)第三个例子</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">        AtomicInteger counter = <span class="keyword">new</span> AtomicInteger();</span><br><span class="line">        ThreadPoolExecutor threadPoolExecutor = <span class="keyword">new</span> ThreadPoolExecutor(</span><br><span class="line">                <span class="number">5</span>, <span class="number">5</span>, <span class="number">1000</span>, TimeUnit.SECONDS,</span><br><span class="line">                <span class="keyword">new</span> ArrayBlockingQueue&lt;&gt;(<span class="number">100</span>),</span><br><span class="line">                (r) -&gt; <span class="keyword">new</span> Thread(r, counter.addAndGet(<span class="number">1</span>) + <span class="string">&quot; 号 &quot;</span>),</span><br><span class="line">                <span class="keyword">new</span> ThreadPoolExecutor.AbortPolicy());</span><br><span class="line"></span><br><span class="line">        CyclicBarrier cyclicBarrier = <span class="keyword">new</span> CyclicBarrier(<span class="number">5</span>,</span><br><span class="line">                () -&gt; System.out.println(<span class="string">&quot;裁判：比赛开始~~&quot;</span>));</span><br><span class="line"></span><br><span class="line">        <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; <span class="number">10</span>; i++) &#123;</span><br><span class="line">            threadPoolExecutor.submit(<span class="keyword">new</span> Runner(cyclicBarrier));</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">static</span> <span class="class"><span class="keyword">class</span> <span class="title">Runner</span> <span class="keyword">extends</span> <span class="title">Thread</span></span>&#123;</span><br><span class="line">        <span class="keyword">private</span> CyclicBarrier cyclicBarrier;</span><br><span class="line">        <span class="function"><span class="keyword">public</span> <span class="title">Runner</span> <span class="params">(CyclicBarrier cyclicBarrier)</span> </span>&#123;</span><br><span class="line">            <span class="keyword">this</span>.cyclicBarrier = cyclicBarrier;</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        <span class="meta">@Override</span></span><br><span class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>&#123;</span><br><span class="line">            <span class="keyword">try</span> &#123;</span><br><span class="line">                <span class="keyword">int</span> sleepMills = ThreadLocalRandom.current().nextInt(<span class="number">1000</span>);</span><br><span class="line">                Thread.sleep(sleepMills);</span><br><span class="line">                System.out.println(Thread.currentThread().getName() + <span class="string">&quot; 选手已就位, 准备共用时： &quot;</span> + sleepMills + <span class="string">&quot;ms&quot;</span> + cyclicBarrier.getNumberWaiting());</span><br><span class="line">                cyclicBarrier.await();</span><br><span class="line"></span><br><span class="line">            &#125; <span class="keyword">catch</span> (InterruptedException e) &#123;</span><br><span class="line">                e.printStackTrace();</span><br><span class="line">            &#125;<span class="keyword">catch</span>(BrokenBarrierException e)&#123;</span><br><span class="line">                e.printStackTrace();</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"><span class="number">5</span> 号  选手已就位, 准备共用时： 152ms0</span><br><span class="line"><span class="number">3</span> 号  选手已就位, 准备共用时： 542ms1</span><br><span class="line"><span class="number">4</span> 号  选手已就位, 准备共用时： 622ms2</span><br><span class="line"><span class="number">1</span> 号  选手已就位, 准备共用时： 783ms3</span><br><span class="line"><span class="number">2</span> 号  选手已就位, 准备共用时： 798ms4</span><br><span class="line">裁判：比赛开始~~</span><br><span class="line"><span class="number">1</span> 号  选手已就位, 准备共用时： 129ms0</span><br><span class="line"><span class="number">4</span> 号  选手已就位, 准备共用时： 149ms1</span><br><span class="line"><span class="number">5</span> 号  选手已就位, 准备共用时： 152ms2</span><br><span class="line"><span class="number">2</span> 号  选手已就位, 准备共用时： 293ms3</span><br><span class="line"><span class="number">3</span> 号  选手已就位, 准备共用时： 768ms4</span><br><span class="line">裁判：比赛开始~~</span><br></pre></td></tr></table></figure>

<p>从例子看出CyclicBarrier实现了 “人满发车”的场景</p>
<h3 id="3、CyclicBarrier应用场景"><a href="#3、CyclicBarrier应用场景" class="headerlink" title="3、CyclicBarrier应用场景"></a>3、CyclicBarrier应用场景</h3><p>从上面三个例子可以总结出 CyclicBarrier适用的场景</p>
<ul>
<li>CyclicBarrier 可以用于多线程计算数据，最后合并计算结果的场景 如例子2所示</li>
<li>利用CyclicBarrier的计数器能够重置，屏障可以重复使用的特性，可以支持类似“人满发车”的场景 如例子3所示</li>
</ul>
<h3 id="4、CyclicBarrier与countDownLatch区别总结"><a href="#4、CyclicBarrier与countDownLatch区别总结" class="headerlink" title="4、CyclicBarrier与countDownLatch区别总结"></a>4、CyclicBarrier与countDownLatch区别总结</h3><ol>
<li>CountDownLatch的计数器只能使用一次，而CyclicBarrier的计数器可以使用reset() 方法重置。所以CyclicBarrier能处理更为复杂的业务场景，比如如果计算发生错误，可以重置计数器，并让线程们重新执行一次</li>
<li>CyclicBarrier还提供getNumberWaiting(可以获得CyclicBarrier阻塞的线程数量)、isBroken(用来知道阻塞的线程是否被中断)等方法。</li>
<li>CountDownLatch会阻塞主线程，CyclicBarrier不会阻塞主线程，只会阻塞子线程。</li>
<li>CountDownLatch和CyclicBarrier都能够实现线程之间的等待，只不过它们侧重点不同。CountDownLatch一般用于一个或多个线程，等待其他线程执行完任务后，再执行。CyclicBarrier一般用于一组线程互相等待至某个状态，然后这一组线程再同时执行。</li>
<li>CyclicBarrier 还可以提供一个 barrierAction，合并多线程计算结果。</li>
<li>CyclicBarrier是通过ReentrantLock的”独占锁”和Conditon来实现一组线程的阻塞唤醒的，而CountDownLatch则是通过AQS的“共享锁”实现</li>
</ol>
<h3 id="5、CyclicBarrier源码跟踪"><a href="#5、CyclicBarrier源码跟踪" class="headerlink" title="5、CyclicBarrier源码跟踪"></a>5、CyclicBarrier源码跟踪</h3><p>跟了三个工具类的源码，自己画了流程图，其中CyclicBarrier只跟源码跟了一遍，还需再看几遍</p>
<p><a target="_blank" rel="noopener" href="https://www.processon.com/view/link/6273e75a0791290711f7b81b">CountDownLatch源码分析</a></p>
<p><a target="_blank" rel="noopener" href="https://www.processon.com/view/link/627529c21efad40df024ea99">Semaphorer源码分析</a></p>
<p><a target="_blank" rel="noopener" href="https://www.processon.com/view/link/62755ac61efad40df0253a9f">CyclicBarrier源码分析</a></p>
 
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